1. Field of the Invention
The present invention relates to a tool clamping device for holding a tool at one end of a main spindle rotatably supported on a headstock.
2. Description of the Prior Art
A tool clamping device of the prior art for clamping a tool on a main spindle is disclosed in Japanese Utility Model Publication No. 55-15942. This known tool clamping device has a drawbar inserted into the spindle hole of a tubular main spindle so as to be slidable along the longitudinal axis of the main spindle, and a holder provided at one end of the drawbar and radially movably holding a plurality of rigid balls. A clamping spring is always seated on a spring seat fixed to the drawbar so as to push the drawbar in a tool clamping direction. An unclamping cylinder for moving the drawbar in a tool releasing direction is provided at the other end of the main spindle.
In this tool clamping device, since the spring seat on which the clamping spring is seated is fixed to the drawbar, the clamping spring is compressed as the drawbar is retracted. Accordingly, in removing a tool from the main spindle, the unclamping cylinder is actuated for releasing the tool to push the drawbar in the tool releasing direction so that the tool is unclamped, after the tool has been gripped by a tool changing arm. Then the tool changing arm is advanced to remove the tool from the main spindle. If the tool is unclamped before the tool changing arm grips the tool, the tool falls off of itself from the nose of the main spindle.
In order to mount a tool on the main spindle, the unclamping cylinder is actuated so as to retract the drawbar after the tool has been inserted into the main spindle by the tool changing arm. After the tool has thus been clamped, the tool changing arm releases the tool. That is, the tool is unclamped after the same has been gripped by the tool changing arm and the tool is clamped after the same has been inserted into the main spindle by the tool changing arm. Accordingly, this known tool clamping device requires a long time period for automatic tool changing operation and the tool changing arm cannot be operated at a high speed.